1. Field
Apparatuses and methods consistent with exemplary embodiments relate to transmissive image modulators, and more particularly, to transmissive image modulators capable of modulating images across a wide bandwidth by employing a stacked diode structure having multi absorption modes.
2. Description of the Related Art
An image captured by using a common camera does not include information regarding a distance between the camera and an object. To embody a 3-dimensional (3D) image capturing apparatus, such as a 3D camera, an additional device for measuring distances from a plurality of points on surfaces of an object is necessary. Distance information with respect to an object may be acquired via a stereo vision method using two cameras or a triangulation method using a structured light and a camera. However, using the above-stated methods, the accuracy of distance information rapidly deteriorates as a distance to an object increases. Furthermore, the above methods depend on surface condition of the object, and thus, it is difficult to acquire precise distance information.
For acquisition of more precise distance information, a time-of-flight (TOF) method has been introduced. In the TOF method, a laser beam is emitted to an object and a period of time until light reflected by the object is received is measured by a light receiving unit. In the TOF method, light of a particular wavelength (e.g., near infrared ray having a 850 nm wavelength) is emitted to an object by using a light emitting diode (LED) or a laser diode (LD), a light of the same wavelength reflected by the object is received by a light receiving unit, and special processes for extracting distance information are performed. Various TOF methods have been introduced. For example, in a direct time measuring method, a pulse light is emitted to an object, a period of time until reception of light reflected by the object is measured by using a timer, and a distance to the object is calculated based on a result of the measurement. In a correlation method, a pulse light is emitted to an object and a distance to the object is measured based on the brightness of light reflected by the object. In a phase delay measuring method, a continuous wave (e.g., sine wave) of light is emitted to an object, a phase difference of light reflected by the object is detected, and the phase difference is converted into a distance to the object.
Furthermore, there are various types of phase delay measuring methods. From among these phase delay measuring methods, an external modulation method for modulating the amplitude of reflected light by using a light modulator and a method of measuring a phase delay by photographing the modulated reflected light by using an imaging device, such as a CCD or a CMOS, are useful for acquiring a high-resolution distance image. Using the external modulation method, a brightness image may be acquired by accumulating or sampling an amount of light incident onto an imaging device for a predetermined period of time, and phase difference and distance to an object may be calculated based on the brightness image. Although a common imaging device may be used as is for external modulation, a light modulator capable of rapidly modulating light at a frequency from dozens of MHz to hundreds of MHz is needed for acquiring a precise phase difference.
Examples of light modulators include an image intensifier or a transmissive light modulator using the Pockel effect or Kerr effect based on crystal optics. However, such light modulators feature large volumes, use high voltage of several kV, and are expensive.
Recently, a GaAs semiconductor-based light modulator, which is relatively easy to embody, is relatively small, and operates at a relatively low voltage, has been introduced. GaAs-based light modulators include a multiple quantum well (MQW) arranged between a P− electrode and an N− electrode and uses a phenomenon that light is absorbed in an MQW when a reverse bias voltage is applied to both PN electrodes. A GaAs-based light modulator features high-speed operation, relatively low driving voltage, and a large ON/OFF reflectivity difference (that is, contrast range). However, a GaAs-based light modulator features very narrow bandwidth from about 4 nm to about 5 nm.
A 3D camera uses a plurality of light sources, and there is slight deviation in center wavelengths of the light sources. Furthermore, the center wavelength of light emitted by a light source may change with temperature. In the same regard, a center absorption wavelength of a light modulator varies according to manufacturing process variables and temperature. Therefore, a light modulator capable of modulating light across a wide bandwidth is necessary for a 3D camera. However, there is a trade-off between an ON/OFF reflectivity difference and bandwidth, and thus, it is difficult to increase ON/OFF reflectivity difference and bandwidth simultaneously.
Meanwhile, since a reflective light modulator reflects modulated light, a light path for providing the modulated light to an imaging device (CCD or CMOS) is complicated, and thus, it is necessary to configure an additional optic system. However, since a transmissive light modulator transmits modulated light, configuration of an optic system may become relatively simple by using a transmissive light modulator. Like in the case of a reflective light modulator, there is also a trade-off between an ON/OFF reflectivity difference and bandwidth in a transmissive light modulator, and thus, it is important to simultaneously increase the ON/OFF reflectivity difference and bandwidth.
To increase the transmissivity difference and bandwidth, a MQW may be formed to have a greater thickness. However, if a thickness of an MQW increases, an driving voltage of a light modulator also increases. If an driving voltage of a light modulator increases, power consumption increases in proportion to the square of the driving voltage. Increased power consumption causes increased heat generated by an image modulator, and thus, the performance of the image modulator may deteriorate due to the generated heat. Therefore, researches are being made for improving the performance of a light modulator with a reduced driving voltage.